Fundamentals of Fluvial Geomorphology and Channel Processes
(cross section shape), planform geometry, and the channel slope. The purpose of this section is to
introduce you to some of the basic characteristics of rivers, and help define some of the confusing
terminology you may encounter when dealing with rivers.
Rivers and streams are dynamic and continuously change their position, shape, and other
morphological characteristics with variations in discharge and with the passage of time. It is important not
only to study the existing river but also the possible variations during the lifetime of the project, particularly
in terms of effective treatment of bank erosion. The characteristics of the river are determined by the water
discharge, the quantity and character of sediment discharge, the composition of the bed and bank material
of the channel, geologic controls, the variations of these parameters in time, and man's activities. To
predict the behavior of a river in a natural state or as affected by man's activities, we must understand the
characteristics of the river as well as the mechanics of formation.
220.127.116.11 Channel Pattern
Channel pattern describes the planform of a channel. The primary types of planform are
meandering, braided, and straight. In many cases, a stream will change pattern within its length. The type
pattern is dependent on slope, discharge, and sediment load.
The most common channel pattern is the meandering stream (Figure 3.3). A meandering channel
is one that is formed by a series of alternating changes in direction, or bends. Relatively straight reaches of
alluvial rivers rarely occur in nature. However, there are instances where a reach of river will maintain a
nearly straight alignment for a long period of time. Even in these relatively straight reaches, the thalweg
may still meander and alternate bars may be formed. Straight streams generally occur in relatively low
energy environments. The braided pattern is characterized by a division of the river bed into multiple
channels (Figure 3.4). Most braided streams are relatively high gradient and relatively coarse streams.
18.104.22.168 Channel Geometry and Cross Section
The following paragraphs describe the channel geometry and cross sectional characteristics of
streams. Since meandering streams are the most common form of alluvial channels this discussion will focus
primarily on this stream type.
Pools and Crossings.
A schematic showing features associated with meanders and
channels is given in Figure 3.5. As the thalweg, or trace formed by the deepest portion of the channel,
changes from side to side within the channel, the momentum of the flow affects the cross-sectional geometry
of the stream. In bends, there is a concentration of flow due to centrifugal forces. This causes the depth
to increase at the outside of the bend, and this area is known as a pool. As the thalweg again changes sides
below a bend, it crosses the centerline of the channel. This area is known as the riffle or crossing. At the
point of tangency between adjacent bends, the velocity distribution is fairly consistent across the cross
section, which is approximately rectangular in shape.